Update monolith-sample.conf

-Replaced undefined monolith value  with server_name  (my.hostname.com) value in reference tho ths issue https://github.com/matrix-org/dendrite/issues/2078

docs/nginx/monolith-sample.conf

@@ -23,6 +23,6 @@ server {
     }
 
     location /_matrix {
-        proxy_pass http://monolith:8008;
+        proxy_pass http://my.hostname.com:8008;
     }
 }